Let's start with some basic notation:
<?php echo $SomeVariable; include('template.php'); ?>
Now let's fire up a server in the terminal:
$ php -S localhost:8000
Take any multi-page assignment from this class and separate your navigation into a single PHP file.
We won't be delving too deeply into PHP. Still I encourage you to dig deeper on your own. The lynda video series PHP with MySQL Essential Training by Kevin Skoglund is a fine place to start. You might create an account on Codecademy and go through the PHP module up to and including functions Part I.